text: Trace concentrations of unstable isotopes of some mononuclidic elements are found in natural samples. For example, beryllium-10 (Be), with a half-life of 1.4 million years, is produced by cosmic rays in the Earth's upper atmosphere; iodine-129 (I), with a half-life of 15.7 million years, is produced by various cosmogenic and nuclear mechanisms; caesium-137 (Cs), with a half-life of 30 years, is generated by nuclear fission. Such isotopes are used in a variety of analytical and forensic applications.